The present invention relates to a structure of a four-cycle engine and, more particularly, to a drive mechanism for a four-cycle engine, which transmits an output of the engine.
Two-cycle engine, which has a relative simple structure and is lightweight, compact, and high-powered, is the mainstream of engines each mounted on a small snow vehicle. However, in recent years, demands for exhaust emission control and improvement of fuel economy against environment problems have promoted the employment of four-cycle engines as those mounted on small snow vehicles.
When employing a four-cycle engine, it is necessary to ensure an output thereof by setting the engine in such a way as to meet high-engine-speed specifications. This is because the four-cycle engine is inferior in output to a two-cycle engine of equal piston displacement. Thus, as shown in Japanese Application Publication Number 11-334393 (especially page 3 or FIG. 1), a drive mechanism for the conventional art four-cycle engine is configured in such a way as to decelerate and transmit power mainly to an output shaft separated from a crankshaft.
Four-cycle engine 200, which is mounted on a small snowmobile and has a V-belt type continuously variable transmission as illustrated in FIG. 6, is concretely described as a conventional art engine.
As shown in FIG. 6, the engine 200 is mounted on the small snowmobile so that an axis of a crankshaft 208 is directed along the direction of width of a vehicle body, that is, along the lateral direction. Further, an output shaft 271, to which an output of the engine 200 is transmitted from the crankshaft 208, is disposed in front of the engine 200 in parallel with the crankshaft 208. Drive clutch 47 of the V-belt type continuously variable transmission is disposed at the left end portion of the output shaft 271. Incidentally, reference numeral 49 designates V-belt for power transmission.
In rear of the engine 200, an oil pump 38 and a water pump 50, to which power is transmitted through a chain 39 from a sprocket 208c provided at an end portion (that is, a left end portion) of the crankshaft 208, are coaxially placed, and disposed in parallel with the crankshaft 208.
The separated output shaft 271 is disposed to the side of one of the end portions (that is, the left-end side) of the crankshaft 208. Flywheel magneto 60 is disposed at the other end portion (that is, the right end portion) of the crankshaft 208. The output shaft 271 is rotatably supported in a front projection portion of a crankcase 205 through bearings 248 and 249. Further, a driving force of a crankshaft 28 is transmitted to the output shaft 271 by causing a driven gear 271a, which is provided at an end portion (that is, a right end portion) of the output shaft 271 in the proximity of the center of the crankshaft 208, to mesh with a drive gear 209 provided in a web in the vicinity of the center of the crankshaft 208.
However, according to the aforementioned conventional art configuration, it is necessary for juxtaposing the output shaft 271 and the crankshaft 208 that the output shaft 271 is off-set frontwardly of the engine to a place at which the output shaft 271 does not interfere with a crank web 210, and that the output shaft 271 is journaled in the bearings.
Further, with the aforementioned conventional art configuration, the engine 200 largely projects in a forward-rearward direction or in the direction of width of the vehicle body. Thus, the conventional art has problems in that the center of gravity of the engine become off-set to the front thereof, and that this affects the steering ability of the vehicle.
Thus, as in the case of an engine 300 shown in FIG. 7, there has been devised a method for avoiding the interference between an output shaft 371 and a crankshaft 308 by placing the output shaft 371 in such a way as to be deviated in a direction (that is, to the left) from a crank web 310 positioned at one end (that is, at the left end) of the crankshaft 308 so as to reduce a part, in which the crank web 310 and the output shaft 271 interfere with each other, in a crankcase 305. However, this method has a problem in that because the output shaft 371 largely projects to one side, the overall width of the engine is large. Incidentally, reference character 371a designates a driven gear provided at the side of the output shaft 371. Reference numeral 309 denotes a drive gear provided at the side of the crankshaft 308.
Furthermore, because a drive clutch 47 attached to the output shaft 371 is placed at the front portion of the engine 300 and far away from a driven clutch (not shown) provided at a drive side in rear of the engine 300, the length of V-belt 49 is large. Thus, the conventional art has problems in that a space for routing the V-belt 49 is needed, and that the manufacturing cost of the engine increases.
On the other hand, even when the output shaft 371 is disposed in a rear portion of the engine 300, there is the necessity for avoiding the interference between the output shaft 371 and the crankshaft 308 by placing the output shaft 371 in such a way as to be off-set rearwardly of the engine 300 to a place at which the output shaft 371 does not interfere with the crank web 310, or by deviating the output shaft 371 to one side from the crank web 310. Thus, problems similar to those in the case of placing the output shaft 371 at the front portion of the engine 300 occur.
Furthermore, when the output shaft 371 is placed at the rear portion of the engine 300, there is caused the need for moving the engine 300 frontwardly in the vehicle so as to avoid the interference between the output shaft 371 and each of a steering post (not shown) and a heat exchanger (or a cooling unit (not shown)) disposed in rear of the engine 300. Therefore, the conventional art has a problem in that the center of gravity of the engine moves frontwardly, and that this adversely affects the steering ability of the vehicle.